An avalanche photodiode is a light detector which typically comprises a .pi.-type conductivity substrate having an N-type region extending a distance into the substrate from a second major surface thereof and a P-type region extending a further distance into the substrate from the N-type region, thereby forming a P-N junction. A P.sup.+ -type conductivity contacting region extends a distance into the substrate from a first major surface thereof to provide a layer suitable for electrical contact. Upon application of a reverse bias voltage to this photodiode the depletion region of the diode, i.e., that region having no free electrical charge, reaches through to the .pi.-type substrate region which the peak electrical field at the P-N junction is between about 5 and 10 percent less than that required to produce an avalanche breakdown. A further increase in the applied voltage causes the depletion region to increase rapidly towards the P.sup.+ -type region, while the electric field throughout the device increases relatively slowly.
Electrical isolation at the first major surface is achieved by separating the P.sup.+ -type region into two separate regions, an inner active region and a neighboring guard-band region. Webb, in U.S. Pat. No. 4,129,878, has disclosed that carrier injection from the first major surface between the inner and outer regions produces injected carriers which increase the noise in the photodiode. Webb also discloses that carrier injection can be reduced by forming a P.sup.- -type region in the gap between the inner and outer regions. With this structure, very high isolation between inner and outer regions is achieved only when sufficient bias voltage is applied to fully deplete the P.sup.- -type region in the gap. Experience has shown, however, that under these conditions if this gap is opposite a region of high gain, carrier generation at the surface produces injection into the high gain region thereby producing additional undesirable noise.
Thus, it would be desirable to have an improved avalanche photodiode structure which reduces the effect of carrier generation and injection from those regions of the surface between the inner and outer P.sup.+ regions by reducing the gain experienced by these injection carriers.